Low emissivity or Low-E or low thermal emissivity windows refers to the surface condition that emits low levels of radiant thermal energy or heat. All materials absorb, reflect, and emit radiant energy.
Emissivity is the value given to materials based on the ratio of heat emitted compared to a black body, on a scale from zero to one. A black body (a theoretical object that absorbs 100% of the radiation that hits it) would have an emissivity of 1 and a perfect reflector would have a value of 0.
Reflectivity is inversely related to emissivity and when added together their total should equal 1 for an opaque material. So the higher the number (between 0 and 1 usually express with two decimal places) for the emissivity of an object, the more it absorbs radiation. Conversely, a low-e material such as aluminum foil has a thermal emissivity value of 0.03 and a thermal reflectance value of 0.97, meaning it reflects 97% of radiant thermal energy and emits only 3%. Low-emissivity window glass is manufactured with metal-oxide coatings.
Lower Low-E ratings translate into more reflected heat. Another factor is the U-Value which is a measure of the heat flow through the windows. The higher the U-Value the more heat flows through so a good U-Value is a low U-Value, as you want to keep heat inside the building or outside depending on the climate you live in. The U-Value physically describes how much thermal energy in Watts [W] is transported through the window with the size of 1 square meter [m2] at a temperature difference of 1 Kelvin [K] (=1°C). Thus the unit for U-values is W/(m^2K).
A better understanding of Low-E windows and how they could enhance your home while lowering energy costs can be gained by calling 208-376-0000 for a free in-home estimate.